The preparation of block copolymers is well known. In a synthetic method an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed resulting in a living homopolymer. To this living homopolymer is added a second monomer which is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block polymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer unreactive toward monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. Alternately, the living block copolymers can be reacted with multifunctional condensing agents commonly referred to as coupling agents. Coupling of the living ends results in radial polymer having at least two arms.
This synthetic approach allows the construction of materials of great practical utility. When the two blocks are sufficiently dissimilar they will not mix but will be microphase separated. This condition is to be distinguished from ordinary phase separation in that the two dissimilar materials are connected through chemical bonds. As such the two blocks become segregated but are not allowed to migrate away from each other. This microphase separated condition may persist in both the solid and melt states. When radial polymers possess such dissimilar blocks which are rubbery and glassy a full range of material characteristics can be achieved, from thermoplastic elastomeric to impact toughened thermoplastic. If the central block is rubbery and the endblocks are glassy then the useful rubbery character of these materials arises from the constrained nature of the rubbery chains. Each end of the rubbery block is anchored in a glassy block. As a result mechanical energy is elastically stored in rubber chain extension when the material is subjected to a bulk deformation.
Since this microphase separated condition commonly exists in both the solid and melt states of radial block copolymers, a mechanism for elastically storing energy exists even in melts. In this case the melt has a significant elastic character. Thus, while the microphase separated state is the feature which gives the solid its useful properties, it also contributes to sometimes exceedingly high melt viscosities. In general, this leads to high energy costs for block copolymer melt processing. In limiting cases, block copolymers alone cannot be melt processed but must have processing aids such as oils and thermoplastic resins incorporated in order to be handled in melt processing equipment. Further the phase separated nature of the material in conjunction with the high viscosities makes melt compounding of these block copolymers with other components difficult. Poor degrees of mixing can result. It is these problems that the present invention addresses.
Recent theoretical work has outlined the thermodynamics controlling the microphase separated character of block copolymers. The thermodynamics disclosures are limited to discussions on generic block copolymer molecules. According to the thermodynamic theory, the state of the block copolymer, microphase separated or homogeneous, is determined by a combination of four variables: the chemical types of the constituent blocks and the resulting thermodynamic interaction between them, the molecular weights of the blocks, the relative amounts of the blocks, and the temperature. Any one variable alone is not sufficient to describe the thermodynamic state of the block copolymer and thereby its resulting morphology and mechanical behavior. The three material variables and temperature must be specified in order to determine the block copolymer's thermodynamic state. A discovery of this invention is the specification of these variables for radial block copolymers of monoalkenyl aromatic hydrocarbons and conjugated diolefins.